1. Field of the Invention
The invention relates to compositions of ethylene/methacrylic acid and acrylic acid copolymers which have improved melt-thermal stability. More particularly, it relates to such copolymers which additionally have small amounts of anhydride or anhydride forming comonomers copolymerized within the polymer chain.
2. Description of Related Art
Copolymers of ethylene and the monocarboxylic acids methacrylic or acrylic acid only are well known. Commercially such dipolymers typically contain at least about 75 weight percent and up to about 92 weight percent ethylene. They are used for molding, packaging and some adhesive applications. Examples of such ethylene/acid copolymers include those sold under the tradename, Nucrel.RTM., which are produced by E. I. du Pont de Nemours and Company. Such ethylene/acid copolymers are described in U.S. Pat. No. 4,351,931 (Armitage).
The above patent does not disclose use of acid comonomers other than acrylic and methacrylic acid, and in practice these monocarboxylic acids are the only commercially important acid comonomers in ethylene/carboxylic acid copolymers. However, other acid comonomers which polymerize with ethylene, and the resulting copolymers have been disclosed, for example, in U.S. Pat. No. 3,264,272 (Rees). Copolymers containing 0.2 to 25 mole percent of dicarboxylic acid comonomers such as itaconic, maleic, fumaric acids, their monoalkyl esters such as methyl hydrogen maleate, and their anhydrides such as maleic anhydride are disclosed. Copolymers containing ethylene and 6 weight percent itaconic acid, and copolymers with 6 weight percent maleic anhydride are exemplified. No copolymers with both monocarboxylic acid and dicarboxylic acid comonomers (or their derivatives) are specifically disclosed, though terpolymers with non-acid termonomers such as alkyl acrylates and methacrylates are disclosed. In this patent, the acid copolymers are precursor polymers for ionomers.
Terpolymers of ethylene, (meth)acrylic acid and alkyl acrylates form a class of acid copolymers with lower modulus and good low temperature properties. A method of preparing these acid copolymers is disclosed in U.S. Pat. No.4,690,981. They are used principally to form `soft` ionomers by neutralization of the acid. However, these acid copolymers with acrylates also find utility in their own right.
Ethylene/(meth)acrylic acid copolymers, including terpolymers with an alkyl acrylate are deficient in melt-thermal stability above about 240.degree. C. Melt flow starts to decrease. This is believed to be due to anhydride formation from two carboxylic acid groups in adjacent chains which form crosslinks, and hence reduce tractability and melt flow. This results in increased gel, decreased melt drawability, and difficult extruder purging after melt processing in an extruder.
Maleic anhydride as a monomer grafted onto existing polymers is well known as a means of obtaining a polar functionality in polyolefin polymers. Such graft-copolymers are useful as compatibilizing agents and as components of hot-melt adhesives. There are also disclosures of maleic anhydride grafted ethylene/monocarboxylic acid copolymers. Typical is JP-83-109721, which discloses laminates where one layer is an ethylene/monocarboxylic acid polymer grafted with 0.05 to 5.0% maleic anhydride. However, grafting requires an additional step after regular polymerization, and in addition is subject to considerable variation in product obtained, as well as changes in properties from ungrafted material, particularly a reduction in melt flow. Reduction in melt flow is the very factor which melt-thermal stabilization seeks to avoid.
The problem of melt-thermal instability has been well recognized, and various attempts have been made to solve it. U.S. Pat. No. 4,594,382 (Hoening et al.), discloses that addition of 5% or less of a hydrated compound which decomposes at from 100 to 300.degree. C., such as hydrated alumina, improves the melt-thermal stability.
A related approach is disclosed in U.S. Pat. No. 5,276,135 (Powell), where allowing a small controlled increase in the (low) moisture content over the amount which normally exists after air and nitrogen purging, improves melt-thermal stability. This patent also discloses that low temperature polymerization of ethylene/(meth)acrylic acid copolymers, below the typical 200-270.degree. C., produces polymer with greater melt-thermal stability. This is said to be due to the greater number of adjacent carboxylic acid units formed when low temperature polymerization is used, adjacent monocarboxylic acid groups reacting together, releasing water and forming intrachain anhydride groups in preference to interchain anhydride groups when no diads are present. However, low temperature polymerization strongly decreases productivity, in addition to making polymer with quite different mechanical properties than polymer made at normal polymerization temperatures.
All these approaches are presumed to be effective because water or released water suppresses interchain anhydride crosslink formation. They have the disadvantages of low productivity, change in the properties of the polymer, or need for a significant amount of a particulate additive.
There is a need for a method of improving the melt-thermal stability of ethylene/(meth)acrylic acid copolymers, including terpolymers with alkyl acrylates, which does not involve (i) reducing productivity during preparation of the copolymer, (ii) a change in mechanical properties, (iii) the difficulty of producing polymer with controlled moisture levels, or (iv) does not require use of an additive.